All animals, including humans, must distinguish between behaviorally important events that require attention and other stimuli that do not. Appropriate sensory gating is critical for processing complex information and for remaining alert during simple, but critical tasks. The ability to selectively attend to relevant stimuli is also critical for effective learning. Conversely, inappropriate sensory gating is an important contributor to cognitive dysfunction associated with several psychiatric and neurological disorders as well as chronic pain. Indeed a number of disorders, including schizophrenia, autism and attention deficit hyperactivity disorder, are associated with deficits in sensory gating and attention. Remarkably, nothing is understood about the molecular basis of attention in any system. We have identified a molecular mechanism at sensory neuron synapses that contributes to sensory gating that mediates an attention-like process in a simple model system, the marine invertebrate Aplysia. We have recently made substantial progress in analyzing the mechanism responsible for switching sensory synapses between active and silent states. This bistable synaptic switch involves homosynaptic depression, in which individual release sites are silenced, and burst dependent protection (BDP) from depression, in which a small burst of action potentials prevents the silencing of release sites. Our recent analysis has implicated classical, calcium-activated protein kinase C (PKC Apl-1) in BDP and the small G protein Arf, together with its upstream regulator GEF, in the silencing of release sites. Through this mechanism, animals remain responsive or attentive to salient stimuli that have biological importance and ignore stimuli that are behaviorally irrelevant. Whereas these changes in sensory signaling are non-associative, animals also alter their responsiveness to importance of stimuli during classical conditioning. Conditioning in Aplysia involves associative plasticity at the same sensory synapses. In our analysis of associative synaptic plasticity, we have cloned and characterized 4 adenylyl cyclase (AC) isoforms expressed in the nervous system of Aplysia. We can now test a novel hypothesis about the contribution of one of these ACs to associative plasticity during conditioning. In Aim 1, we will test the roles of specific Arf GEFs (that catalyze GTP binding to and activating Arf) and specific Arf isoforms in silencing sensory neuron synapses. In Aim 2 we will use study the precise colocalization of these signaling molecules, including PKC Apl-1, a scaffolding protein PICK1 and Arf GEF, with calcium channels at release sites. To image single molecules we will use novel culture techniques in combination with photoactivatable fluorescent tags. In Aim 3, we will test a new hypothesis that calcium- inhibited AC also contributes to requirements for stimulus pairing during initiation of synaptic plasticity.